1. Field of the Invention
This invention relates to a process for forming a transparent conductive film superposed on a substrate by means of a sputtering system or the like, a process for producing a photovoltaic device, a transparent conductive film formed by that process, and a photovoltaic device making use of the transparent conductive film.
2. Related Background Art
Photovoltaic devices comprised of hydrogenated amorphous silicon, hydrogenated amorphous silicon germanium, hydrogenated amorphous silicon carbide, microcrystalline silicon, polycrystalline silicon or compound semiconductor are provided with reflecting films on the back sides of devices in order to improve collection efficiency at long wavelengths. It is desirable for such reflecting films to exhibit reflection performance effective at wavelengths which are near to energy band ends of semiconductor materials and at which their absorption is small, i.e., wavelengths of from 800 nm to 1,200 nm. Those which can well satisfy this condition are metals such as gold, silver, copper and aluminum.
An uneven layer which is optically transparent within a stated range of reflection is also provided in some cases in order to confine in a semiconductor active layer the light having entered a photovoltaic device. Such an uneven layer is commonly provided between the reflecting film formed of any of the above metals and the semiconductor active layer so that the reflected light can effectively be utilized to improve short-circuit current density Jsc. As methods by which such a transparent conductive film used as a light-confining layer is made to have an uneven surface profile, the following techniques are conventionally known.
Japanese Patent Application Laid-Open No. 63-456 discloses that, in a method of forming a film of a polycrystalline metal oxide by vacuum deposition, a film with uneven surface structure can be formed at the surface of a transparent conductive film where a metal oxide material is vacuum-deposited at a rate of 15 to 30 angstroms/second with heating.
Japanese Patent Application Laid-Open No. 8-32094 discloses that a highly reliable solar cell can be obtained where a transparent conductive film consists of at least two layers and film formation temperature and/or film formation rate is/are controlled so that one layer of them is formed as a crystalline transparent conductive film and one layer as an amorphous transparent conductive film.
Japanese Patent Application Laid-Open No. 10-310862 discloses that a reflecting film and a transparent conductive film which have the desired textural structure, have a high reflectance, are inexpensive and have a high reliability can stably be obtained where a film-forming chamber is once heated and then cooled, thereafter a metallic reflecting film is formed on a substrate, and, after active oxygen is brought into contact therewith, a transparent conductive film is formed thereon.
Japanese Patent Application Laid-Open No. 11-220154 discloses that, after a metallic reflecting film is formed on a substrate, a first transparent conductive film is formed in an atmosphere containing 50% or more of oxygen and a second transparent conductive film is formed in an atmosphere of argon and at a temperature higher than that at the time of the formation of the first transparent conductive film, so that in the first transparent conductive film the c-axes of crystal grains stand parallel to the substrate and in the second transparent conductive film the c-axes of crystal grains stand vertical to the substrate, thus a transparent conductive film the surface unevenness of which is free of any ridges can be formed and a solar cell improved in photovoltaic characteristics and having a high reliability can be obtained.
Japanese Patent Application Laid-Open No. 6-116722 discloses that a metal layer and a transparent conductive film are continuously formed by sputtering on a continuous substrate while it is moved (a roll-to-roll method).
Thus, deposited films having a certain good quality can be obtained by regulating the rate of film formation and forming the transparent conductive film in multiple layers.
However, in order to mass-produce good transparent conductive films by spurring at a low cost, there are problems as stated below which should be settled.
In order that the transparent conductive film used as a light-confining layer is grown to have an uneven surface profile which is effective for the characteristics of photovoltaic devices, the film has had to be formed by deposition in an extremely large film thickness and/or at an extremely low rate of film formation. Such a method of forming thin films has problems of a high material cost and a long film formation time, resulting in a very high production cost for the photovoltaic devices. Also, it is difficult to provide any good uneven surface profile only by simply making the film formation rate higher, undesirably bringing about an unstable sputtering discharge and a low film quality.